NEW YORK (Reuters Health) - In a new study from Chile, infants who already had high levels of hemoglobin proteins in their blood and were fed iron-fortified formula ended up with lower scores on tests of thinking and memory than those given low-iron formula.

Researchers wrote in the Archives of Pediatrics & Adolescent Medicine that most infants didn't show any developmental harm as a result of fortified formula. And those who started with low levels of hemoglobin, an iron-rich molecule in red blood cells, did better in the long run if they got the extra iron.

Through the worse scores in some kids were "totally unexpected," study author Dr. Betsy Lozoff said, "It's a very small proportion of the children... I am very cautious about it."

Iron deficiency is recognized as a global public health problem, and low iron can stunt brainpower in kids.

"We know that iron is essential for brain development. The last thing I would want is for people not to give their babies iron," Lozoff, from the University of Michigan in Ann Arbor, told Reuters Health.

KNOW YOUR BABY'S STATUS

Recommendations call for babies in the U.S. to get an iron test during the first year of life to check for deficiencies.

Based on the new findings, other nutrition experts said that if those levels are higher than normal, parents using formula might want to consider one that has some iron, but not too much.

"In general parents think, 'Oh, fortification, that's good,'" said Chessa Lutter, regional advisor on food and nutrition for the Pan American Health Organization, part of the World Health Organization, who wasn't involved in the study.

The WHO recommends breastfeeding exclusively for the first six months of life, continued for two years or longer. Still, "If I were to give formula, I would certainly want to know my baby's iron status," Lutter told Reuters Health.

The findings come from a study originally done in Santiago in 1991 through 1994. Researchers tested babies' hemoglobin levels at age six months and those who weren't anemic were randomly assigned to get either low-iron or high-iron formula until they were a year old.

The high-iron formula contained 12.7 milligrams of iron for every liter of formula -- about the same amount that's in iron-fortified formula sold in the U.S. (Formula in Europe typically contains only about half that much iron.)

Ten years later, Lozoff and her colleagues brought 473 of the original 835 infants back for a suite of thinking, coordination and memory tests.

On average, kids who were given extra iron as babies had slightly lower scores on the tests, including ones measuring hand-eye coordination and spatial reasoning.

When the researchers divided kids up into those that had high and low hemoglobin levels before getting the formula, they found that the extra iron gave a slight boost to kids with initially low levels -- but was linked to lower scores in kids who started with plenty of hemoglobin.

For example, among babies who started with high hemoglobin levels, average IQ at age 10 was 83 in those who were given the high-iron formula, compared to 95 in those who got the less-fortified formula.

BASED ON A SMALL SAMPLE SIZE

The researchers cautioned that the calculation was based on a small number of kids -- only 11 to 13 in each formula group -- and most of the babies they studied started with normal or low hemoglobin.

It's also possible that other aspects of health unrelated to iron, like moms' smoking or kids' medical conditions, were to blame for both the extra-high hemoglobin levels and the lower test scores in some kids.

For example, low oxygen in the bloodstream while babies are in the womb could be toxic to the brain and cause high hemoglobin levels at the same time, said Dr. Michael Georgieff, a child development researcher at the University of Minnesota in Minneapolis who has collaborated with the authors but wasn't involved in this study.

"Maybe there is something about these kids that gave them this high hemoglobin to start with," he told Reuters Health. The other explanation, Georgieff said, is "there's something toxic about getting that much iron in your diet."

WHAT SHOULD PARENTS DO?

Georgieff said that because the study was small, and the 10-year test scores weren't its original focus, the findings need to be confirmed with future research before any conclusions can be made about the possible harms of getting too much iron.

For now, parents can talk with their pediatrician about their own infant's iron status and, if they're no longer exclusively breastfeeding, find out what the best type of formula would be, Georgieff added.

"If you have an abnormally high hemoglobin (level), you probably don't want to be on a high-iron formula," he said. Still, most kids who have normal levels will be fine getting iron-fortified formula, he emphasized.

Lozoff, on the other hand, said that "it's just too premature" for parents to be worried about their babies getting too much iron based on her findings.

"Such an important thing as what you feed your baby should not change based on a single study," she said.